Aluminium alloys and, more particularly, 3xxx alloys are now very widely used in the manufacture of heat exchanger components for the automotive and heating ventilation and air conditioning and refrigeration (HVAC
R) industries due to their combination of good corrosion resistance, formability, thermal stability and amenability to brazing. The heat exchanger components can be, without being limitative, rolled finstock, seam welded tubing from rolled sheet, as well as extruded tubes and profiles. The heat exchangers are typically assembled by furnace or flame brazing and mechanical connections.
For extrusions used in structural applications, 6xxx alloys, containing magnesium (Mg) and silicon (Si) as major alloying elements, are preferred since they have relatively good extrudability and benefit from precipitation hardening whereas the strength of current 3xxx alloys is relatively limited. However, the 6xxx alloys are difficult to braze. Furthermore, for applications where heavy cold forming is required, 6xxx alloys are typically used in the T4 temper which can undergo natural ageing. Thus, strength of 6xxx alloys increases with time after extrusion causing variable springback during bending. The 3xxx alloys are classed as non heat treatable and do not exhibit natural ageing and generally have better extrudability than the 6xxx. Therefore, several attempts have been made to improve the strength of 3xxx alloys without the addition of magnesium and in which the deliberate additions of magnesium are avoided because magnesium can be detrimental to extrudability.
U.S. patent application No. 2007/017605 describes one attempt to improve the strength of the aluminium alloy without the addition of magnesium. In the aluminium alloy, the ratio of manganese (Mn) content to silicon (Si) content was kept between 0.7 and 2.4. The extrusion billet is subjected to homogenization which includes a first-stage heat treatment in which the billet is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the billet is cooled to 400 to 500° C. and maintained at that temperature for three hours or more. The billet is then heated at 480 to 560° C. before being extruded into multiport tubing. When manufacturing multi-port tubing, the extrusion ratio reaches several hundred to several thousand. Moreover, these prolonged heat treatments are energy intensive and time consuming.
The U.S. Aluminium Association (hereinafter called “AA”) alloy 3003 (0.05 to 0.20 wt % of Cu, less than 0.6 wt % of Si, less than 0.7 wt % of Fe, 1.0 to 1.5 wt % of Mn, less than 0.10 wt % of Zn, and the balance Al) is a widely used 3xxx alloy which has many uses including extruded tubing. Tube stock can be drawn to improve mechanical properties but with associated costs. Moreover, tube stock drawing is difficult to achieve on more complex shapes such as automotive crash structures. When extruded, the original grain structure in the billet recrystallizes and the final product typically has a fine-grain structure.
The challenge is therefore to develop an aluminium extrusion that retains the advantageous properties of good corrosion resistance and formability provided by 3xxx alloys while, at the same time, improving its mechanical properties so that it can be used in applications requiring higher strength.